Capacitors are conventionally employed in analog circuits. Capacitors are used in radios, cellular phones, in other RF (radio frequency) applications which require oscillator circuits, as well as in amplifier network ICs. Capacitors are not generally used in ICs dedicated solely to logic circuitry. Consequently, capacitors are less likely to be employed in digital circuits.
However, the majority of IC fabrication processes are dedicated to the fabrication of digital logic circuits. Accordingly, processes for fabricating digital circuitry are well developed. Advances in processing of digital circuitry have enabled significant improvements in miniaturization, operating speed, and packing density.
Since processes for fabricating digital circuitry are widespread, such processes are attractive for use in the fabrication of analog circuits. Accordingly, processes designed for fabricating digital circuitry are increasingly being employed to produce analog circuits. In particular, MOS (metal oxide semiconductor) transistors are being employed in analog circuits instead of bipolar junction transistors.
Typically, capacitors are produced on a semiconductor wafer by forming a p-n junction comprising p-doped and n-doped semiconductor regions. Such a capacitor formed from a p-n junction includes regions of doped semiconductor and associated depletion regions which are influenced by the application of an applied voltage. Consequently, these capacitors disadvantageously have a capacitance which varies with applied voltage.
Alternatively, capacitors may be produced on a semiconductor wafer by forming an MOS structure comprising a metal, a dielectric spacer, and doped semiconductor such as doped polysilicon or diffusion. Disadvantageously, a capacitor constructed using metal and polysilicon has a parasitic capacitance to ground which is larger than the capacitance of the MOS structure. A capacitor constructed using metal and diffusion has a parasitic capacitance to ground which varies with applied voltage.
A capacitor can also be created on a semiconductor wafer which is analogous to a parallel plate capacitor by forming two layers of metal wherein the layers of metal are separated by dielectric spacer. Advantageously, a capacitor comprising two layers of metal separated by a dielectric spacer has a capacitance which does not vary with applied voltage. Such a capacitor, however, has a parasitic capacitance to ground that is similar in magnitude to the capacitance between the two layers of metal.
Thus, there remains a need for a capacitor for analog applications which does not have a capacitance which varies with applied voltage, which has a parasitic capacitance to ground which is small, and which can be formed using fabrication processes conventionally employed to fabricate digital circuitry.